I. Field of the Invention
This invention relates to color change devices, i.e. devices which undergo a change of color when physically disturbed in some way. More particularly, the invention relates to laminated color change devices capable of undergoing a change of color by means other than direct delamination of the constituent layers of the device.
II. Description of the Prior Art
In our prior U.S. Pat. No. 4,837,061 to Smits et. al. issued on Jun. 6, 1989 (the disclosure of which is incorporated herein by reference), a process for producing color change devices, particularly those used as tamper evident structures, is disclosed. The process involves anodizing a color generating metal, such as a valve metal (e.g. Ta, Nb, Zr, Hf and Ti), a refractory metal (e.g. W, V and Mo), a grey transition metal (e.g. Ni, Fe and Cr), a semi-metal (e.g. Bi) or a semiconductor metal (e.g. Si), in order to form an anodic film of oxide having a thickness in the order of the wavelength of light (referred to as an "optically thin" film) intimately contacting the color generating metal. The resulting laminates exhibit a strong interference color when illuminated with white light because of light interference effects between reflections from the closely spaced metal and oxide surfaces and because of light absorption which takes place at the metal/oxide interface when color generating metals are employed.
The resulting structures can be formed as color change devices if the anodization is carried out in an electrolyte containing an adhesion reducing agent, such as a fluoride, which lowers the normally tenacious adhesion of the oxide film to the metal substrate. This allows the oxide film to be detached from the substrate with consequent destruction or modification of the exhibited color. Re-attachment of the oxide layer does not result in regeneration of the original color, so the color change is essentially irreversible and forms an effective indication of tampering.
The detachment of the anodic film from the metal substrate can be assisted by adhering a transparent or translucent layer to the anodic film and using this layer to reinforce the delicate anodic film so that the film can be reliably detached from the metal substrate in large pieces without disintegrating.
While these prior color change devices have proven to be most effective, they are vulnerable to defeat to some extent when used in certain ways. In particular, when the devices are formed as thin flexible strips or sheets to be adhered to an article to be protected by a layer of adhesive or the like (referred to as tamper-evident labels), it may be possible to remove the entire device from the article without detaching the anodic film from the substrate metal and hence without producing a tell-tale color change. A device removed in this way could be reattached to the original article (e.g. a container that had been opened) or attached to a different (e.g. counterfeit) article. Tamper-evident labels of this kind are extremely useful in practice and it would be a considerable advantage to make them more secure.